The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Embodiments are related in general to equipment for servicing subterranean wells. Particularly, the invention relates to an apparatus and methods for controlling the direction and rate at which fluids flow during the primary cementation of a subterranean well.
During a primary cement job, fluids of various densities are circulated through the tubulars, the annular region between the tubulars and the borehole wall, and sometimes the annular regions between two tubular bodies. Most of the time, fluids first travel through the interior of the tubulars. Upon exiting the tubulars, the fluids travel through the annular region between the exterior surface of the tubulars and the borehole wall. Fluids may flow in the opposite direction should operators choose a procedure known in the art as “reverse cementing.” The tubulars may include drill pipe, casing, liner and coiled tubing. Hereinafter, the common term “casing” shall be used to describe a tubular body.
Typically, each fluid is heavier (higher in density) than its predecessor. For example, a spacer fluid is usually heavier than the drilling fluid, and a cement slurry is usually heavier than the spacer fluid. This density hierarchy helps minimize commingling between fluids as they circulate in the well. The density difference also promotes efficient removal of drilling fluid, providing clean casing- and borehole-wall surfaces to which the cement may bond and provide zonal isolation.
A potential consequence of the fluid-density hierarchy is a phenomenon known in the art as free fall or “U-tubing.” The fluids inside the casing and the annulus will naturally tend to achieve a hydrostatic equilibrium. When a heavier fluid such as a cement slurry is introduced inside the casing, a hydrostatic imbalance is created between the casing interior and the annulus. As a result, the cement slurry has a tendency to free fall and draw a vacuum inside the upper part of casing interior. Practitioners of the art will of course recognize that the free-fall tendency may be lessened by friction pressures inside and outside of the casing.
Nevertheless, during many cementing operations, the pump rate into the casing is insufficient to keep the casing full during the early part of the job. This results in a net flow or efflux of fluid out of the annulus. The rate of efflux may be much higher than the inward flow. Eventually, as hydrostatic pressure equilibrium is approached, the rate of efflux from the well falls below the inward-flow rate, and the casing interior gradually refills.
Those skilled in the art recognize that optimal cementing results may not be obtained unless the fluid-flow rate in the well is controlled. Owing to the fluids' rheological properties, an annular-flow rate that is too high or too low may cause poor drilling-fluid removal and compromise zonal isolation.
If a lower-density displacement fluid follows the cement slurry, a second U-tubing event may occur, but in the opposite direction. Cement slurry would re-enter the casing interior, causing a situation known as “cement left in pipe” or CLIP. In addition the cement slurry may no longer cover the annulus across a producing interval, resulting in the loss of zonal isolation.
Hydrostatic imbalances in the well also have implications in the context of foamed cements. When pumping foam there is no free fall per se because the pressure cannot fall to zero at the wellhead. Nevertheless, as the casing-interior pressure falls, the gas volume in the foam (i.e., foam quality) will increase. The foam may collapse if the foam quality reaches the point of instability.
The beginning and end of U-tubing events may be detected by measuring the surface pressure during the cement job. Considering the importance of annular-fluid velocities and pressures to the safe and successful execution of a cement job, it is clear that U-tubing must be considered in any job design. Algorithms exist that permit engineers to simulate the phenomenon.
The well-cementing industry has introduced techniques and devices that address the U-tubing phenomenon. One technique is to control the “back-side” or annular pressure, thereby counterbalancing the internal-casing pressure and reducing free fall. However, this is often not practical, especially in remote locations or if the required back-side pressure is excessive.
Various devices for controlling fluid-flow in a subterranean well have been described (see for example U.S. Pat. No. 5,092,406; U.S. Pat. No. 5,131,473; U.S. Pat. No. 6,520,256; and US 2006/0000993). The devices include downhole chokes and an apparatus that forces fluids to travel through a tortuous path. These devices control the rate at which fluids pass through them, thereby controlling the flow rate in the casing and the annulus. The devices are premounted on or inside the casing string. Once the casing is lowered into the well, fluid-flow control is immediately limited.
Despite the valuable contributions of the prior art, a need remains for operators to freely circulate fluids after the casing is lowered in the well; for example, to condition the annulus and remove gelled drilling fluid that may be coating the exterior casing wall and the borehole wall. The presence of gelled drilling fluid in the annulus is detrimental to achieving a successful primary cementing job. At higher flow rates, hole conditioning is generally more efficient.
In addition, circulating at higher rates may be essential to maintain well control if for example (1) the casing collapses; (2) the surface pressure becomes too high for the cement head; or (3) the hydraulic-horsepower limit of the pumps is reached.
It would also be desirable to delay the maximum-fluid-rate decision until just before the cement job takes place. Such a feature would allow operators to make last minute slurry-density or fluid-composition adjustments in response to current well conditions.